Optical delay lines for electrical skew compensation

ABSTRACT

A skew compensation apparatus and method. In an optical system that uses optical signals, skew may be generated as the optical signals are processed from an input optical signal to at least two electrical signals representative of the phase-differentiated optical signals. A compensation of the skew is provided by including an optical delay line in the path of the optical signal that does not suffer the skew (e.g., that serves as the time base for the skew measurement). The optical delay line introduces a delay T skew  equal to the delay suffered by the optical signal that is not taken as the time base. The two signals are thereby corrected for skew.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S.provisional patent application No. 62/118,420 filed Feb. 19, 2015, andco-pending U.S. provisional patent application No. 62/132,742 filed Mar.13, 2015, each of which applications is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

The invention relates to systems and method for controlling signalpropagation in dual-polarization coherent communication systems ingeneral and particularly to skew compensation in such systems.

BACKGROUND OF THE INVENTION

FIG. 1 is a schematic block diagram 100 of the electrical and opticalcomponents of a prior art coherent optical transceiver. Skew can beintroduced in the traces between elements as well as within eachelement. It would be advantageous for each element to compensate forskew.

In dual-polarization coherent communication, there are at least foursignal paths from the digital signal processor (DSP) to the outputoptical signal. These are the in-phase and quadrature modulator inputsfor X- and Y-input optical polarizations. X- and Y-polarizations areorthogonal polarizations in the input optical fiber. In such a scenario,it is important that the relative timing skew between each of thesesignal paths from the DSP to the output optical signal is kept as low aspossible. There is skew between X- and Y-polarizations, as well asbetween the In-phase and Quadrature components of a signal within acertain polarization. These are called XY and IQ timing skews,respectively. Similarly, there are four such paths from the incomingoptical signal to the DSP. There can be both XY and IQ timing skew inthe transmitter and in the receiver.

There is a need for improved systems and methods for correcting skew.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a skew compensationapparatus, comprising: a signal converter selected from the group ofsignal converters consisting of a signal converter that is configured toconvert at least two optical signals into at least two electricalsignals and a signal converter that is configured to convert at leasttwo electrical signals into at least two optical signals; a first one ofthe at least two electrical signals subject to a delay of magnitudeT_(skew) relative to a second one of the at least two electricalsignals; the signal converter having at least two optical ports and atleast two electrical ports, the at least two optical ports selected fromthe group consisting of at least two input ports and at least two outputports, and the at least two electrical ports selected from the othertype of port in the group consisting of at least two input ports and atleast two output ports; and at least one optical delay line in opticalcommunication with at least one of the at least two optical ports, theat least one optical delay line configured to apply a correctioncomprising a compensation delay to a selected one of the first one andthe second one of the at least two electrical signals so that after thecorrection, the time delay between the first one and the second one ofthe at least two electrical signals is different from T_(skew).

In one embodiment, the delay after correction is less than T_(skew).

In another embodiment, the delay after correction is greater thanT_(skew).

In yet another embodiment, the at least two optical signals are phasedifferentiated and comprise an I component and a Q component.

In still another embodiment, the at least two optical signals areconverted from orthogonal polarizations in an optical carrier.

In a further embodiment, the at least one optical delay line is a singlemode waveguide.

In yet a further embodiment, the at least one optical delay line is amulti-mode waveguide.

In an additional embodiment, the at least one optical delay line has anadjustable optical path length.

In one more embodiment, the adjustable optical path length is configuredto be thermally adjustable.

In still a further embodiment, the adjustable optical path length isconfigured to be adjustable by charge carrier concentration.

In another embodiment, the at least one optical delay line comprisessilicon.

In yet another embodiment, the at least one optical delay line is aswitched delay line.

In still another embodiment, the at least one optical delay line is alxN electro-optic switch combined with N waveguides having differentlengths.

In a further embodiment, the skew compensation apparatus furthercomprises a thermal measurement device and a heater adjacent the opticaldelay line.

In yet a further embodiment, the skew compensation apparatus isconfigured to operate using an optical signal having a wavelength withinthe range of a selected one of an O-Band, an E-band, a C-band, anL-Band, an S-Band and a U-band.

According to another aspect, the invention relates to a method ofcompensating skew, comprising the steps of: providing an apparatus,comprising: a signal converter selected from the group of signalconverters consisting of a signal converter configured to convert atleast two optical signals into at least two electrical signals and asignal converter that configured to convert at least two electricalsignals into at least two optical signals; a first one of the at leasttwo electrical signals subject to a delay of magnitude T_(skew) relativeto a second one of the at least two electrical signals; the signalconverter having at least two optical ports and at least two electricalports, the at least two optical ports selected from the group consistingof at least two input ports and at least two output ports, and the atleast two electrical ports selected from the other type of port in thegroup consisting of at least two input ports and at least two outputports; and at least one optical delay line in optical communication withat least one of the at least two optical port, the at least one opticaldelay line configured to apply a correction comprising a compensationdelay to a selected one of the first one and the second one of the atleast two electrical signals so that after the correction, the timedelay between the first one and the second one of the at least twoelectrical signals is different from T_(skew); and applying at least twoinput signals to the at least two input ports of the signal converter;and applying the compensation delay to a selected one of the first oneand the second one of the at least two electrical signals so that thetime delay between the first one and the second one of the at least twoelectrical signals is different from T_(skew).

In one embodiment, the delay after correction is less than T_(skew).

In another embodiment, the delay after correction is greater thanT_(skew).

In yet another embodiment, the method of compensating skew furthercomprises the step of determining the magnitude T_(skew) of the delay.

In still another embodiment, the at least two optical signals arephase-differentiated and comprise an I component and a Q component.

In a further embodiment, the at least two optical signals are convertedfrom orthogonal polarizations in the optical carrier.

In yet a further embodiment, the at least one optical delay line is asingle mode waveguide.

In an additional embodiment, the at least one optical delay line is amulti-mode waveguide.

In one more embodiment, the at least one optical delay line has anadjustable optical path length.

In still a further embodiment, the adjustable optical path length isconfigured to be thermally adjustable.

In one embodiment, the adjustable optical path length is configured tobe adjustable by charge carrier concentration.

In another embodiment, the at least one optical delay line comprisessilicon.

In yet another embodiment, the at least one optical delay line is aswitched delay line.

In still another embodiment, the at least one optical delay line is a1×N electro-optic switch combined with N waveguides having differentlengths.

In a further embodiment, the method of compensating skew in an opticalsystem further comprises a thermal measurement device and a heateradjacent the optical delay line.

In yet a further embodiment, the input optical signal has a wavelengthwithin the range of a selected one of an O-Band, an E-band, a C-band, anL-Band, an S-Band and a U-band.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a schematic block diagram of the electrical and opticalcomponents of a prior art coherent optical transceiver.

FIG. 2A is a graph of XI and XQ signal waveforms versus time asinitially provided with zero skew.

FIG. 2B is a graph of XI and XQ signal waveforms versus time with skewintroduced, showing how one defines or measures T_(skew).

FIG. 3A is a diagram that illustrates skew compensation within apackaged optical receiver using electrical wiring traces that havedifferent lengths.

FIG. 3B is a diagram showing an embodiment of the electronic componentsin the packaged optical receiver of FIG. 3A.

FIG. 4 is a diagram that shows an embodiment of the reduced package sizeenabled by moving skew compensation from the electrical domain to theoptical domain, according to principles of the invention.

FIG. 5 is a diagram that shows an embodiment of the reduced package sizeenabled by moving skew compensation from the electrical domain to theoptical domain. The optical domain introduces timing skew thatpre-corrects for the skew introduced later.

FIG. 6 is a schematic diagram of an embodiment in which a signal thatrequires a skew compensation which may vary with time or with frequencycan be skew pre-compensated using one of N different skew compensationelements.

FIG. 7 is a schematic diagram that illustrates the general principles ofthe invention.

FIG. 8 is a schematic diagram of an embodiment of a system having Nchannels in parallel in which skew compensation in provided in atransmitter.

FIG. 9 is a schematic diagram of an embodiment having N channels inparallel in which skew compensation in provided in a receiver.

FIG. 10 is a schematic diagram of an embodiment in which N channels eachhaving 4 electrical inputs and 4 electrical outputs are corrected forskew in a skew compensating module comprising a polarizing beamsplitter,one or more hybrid mixers, the skew compensation elements and one ormore optoelectronic converters.

DETAILED DESCRIPTION Acronyms

A list of acronyms and their usual meanings in the present document(unless otherwise explicitly stated to denote a different thing) arepresented below.

-   -   AMR Adabatic Micro-Ring    -   APD Avalanche Photodetector    -   ARM Anti-Reflection Microstructure    -   ASE Amplified Spontaneous Emission    -   BER Bit Error Rate    -   BOX Buried Oxide    -   CMOS Complementary Metal-Oxide-Semiconductor    -   CMP Chemical-Mechanical Planarization    -   DBR Distributed Bragg Reflector    -   DC (optics) Directional Coupler    -   DC (electronics) Direct Current    -   DCA Digital Communication Analyzer    -   DRC Design Rule Checking    -   DUT Device Under Test    -   ECL External Cavity Laser    -   FDTD Finite Difference Time Domain    -   FOM Figure of Merit    -   FSR Free Spectral Range    -   FWHM Full Width at Half Maximum    -   GaAs Gallium Arsenide    -   InP Indium Phosphide    -   LiNO₃ Lithium Niobate    -   LIV Light intensity(L)-Current(I)-Voltage(V)    -   MFD Mode Field Diameter    -   MPW Multi Project Wafer    -   NRZ Non-Return to Zero    -   PIC Photonic Integrated Circuits    -   PRBS Pseudo Random Bit Sequence    -   PDFA Praseodymium-Doped-Fiber-Amplifier    -   PSO Particle Swarm Optimization    -   Q Quality factor

$Q = {{2\pi \times \frac{{Energy}\mspace{14mu} {Stored}}{{Energy}\mspace{14mu} {dissipated}\mspace{14mu} {per}\mspace{14mu} {cycle}}} = {2\pi \; f_{r} \times {\frac{{Energy}\mspace{14mu} {Stored}}{{Power}\mspace{14mu} {Loss}}.}}}$

-   -   QD Quantum Dot    -   RSOA Reflective Semiconductor Optical Amplifier    -   SOI Silicon on Insulator    -   SEM Scanning Electron Microscope    -   SMSR Single-Mode Suppression Ratio    -   TEC Thermal Electric Cooler    -   WDM Wavelength Division Multiplexing

FIG. 2A is a graph 200 of XI and XQ signal waveforms versus time asinitially provided with zero skew. In FIG. 2A there are shown a waveform202 which can be the XI component and a waveform 204 which can be the XQcomponent of a dual-polarization coherent communication system.

FIG. 2B is a graph 201 of XI and XQ signal waveforms versus time withskew introduced, showing how one defines or measures T_(skew). In FIG.2B, the waveform 202 is illustrated as being in the same relative timerelation that it had in FIG. 2A. One can understand this as usingwaveform 202 as a time baseline. However, waveform 204′ is displaced intime relative to waveform 202, as indicated by the curved arrow 210,which displacement is called the skew. The time T_(skew) 220 which is ameasure of the amount of displacement is the difference between therelative time offset T₀ 205 in FIG. 2A and the relative time offset T₁206 in FIG. 2B. T_(skew) 220 may be represented by the relation

T _(skew)=Absolute value (T ₁ −T ₀).

The skew in FIG. 2A is illustrated for the XQ component relative to theXI component. However, in real systems it is possible to have skewdefined as the offset of the XI component relative to the XQ component(e.g., the XQ component is used as the time base for measurement). Insimilar fashion, the YI and YQ components of the dual-polarizationcoherent communication system can also exhibit skew, using either the YIcomponent or the YQ component as the time base for measurement.

The skew is compensated by applying a delay of magnitude T_(skew) to thesignal that is not skewed, so that both signal in a pair of signals XI.XQ and YI, YQ have equal delays, and are therefore in the original timerelation that existed prior to the optical to electrical conversion. Thecompensation is applied in the optical domain as a compensationaccording to the principles of the invention, rather than in theelectrical domain as a post-compensation relative to the optical toelectrical conversion.

FIG. 3A is a diagram 300 that illustrates skew compensation within apackaged optical receiver using electrical wiring traces 310 that havedifferent lengths. In the embodiment illustrated, significant area isneeded to time-align the signals from a set of transimpedance amplifiers(TIAs) to the electrical pins on the package. The optical receivercomponents are indicated by the block 320, which is shown in greaterdetail in FIG. 3B. The optical receiver has a signal input 302 that canreceive an optical input signal. In some embodiments the optical inputsignal is part of a dual-polarization coherent communication system. Theoptical receiver has a local oscillator 304, for example a laser or alaser diode, which provides a frequency signal.

Each component in the signal paths adds some skew to the signal. Thisamount of skew should be minimized. The size of electro-optical modulesimplementing dual-polarization IQ modulators and receivers is affectedby the amount of space needed to compensate for electrical skews.Electrical delays are needed in order to fan-out the electrical tracefrom some small component, such as an amplifier, to the pins on apackage surrounding the device. As can be clearly seen, significant areais required for electrical skew compensation. It is well known that afoot (approximately 30 centimeters) of electrical wiring adds a delay ofapproximately one nanosecond to an electrical signal. Therefore,depending on the amount of skew (e.g., the value of T_(skew)) that hasto be compensated by delaying the components that have not sufferedskew, wiring of significant length may be required.

FIG. 3B is a diagram showing an embodiment of the electronic componentsin the packaged optical receiver 320 of FIG. 3A. As shown in theembodiment of FIG. 3B, the input signal enters at port 302. A smallportion of the input optical signal (typically less than 5%) is splitoff and sent to photodiode 314, which generates an electrical signalthat can be used to monitor properties of the input optical signal, suchas its power content. In other embodiments, the power can be monitoredusing different hardware. The remainder of the input optical signal issent through a variable optical attenuator 312, which can adjust thesignal intensity, and is split by a polarized beam splitter (PBS) 315into x-polarized (X-Pol) and y-polarized (Y-Pol) components. The X-Polcomponent is sent to a 90 degree hybrid mixer 306, and the Y-Polcomponent is sent to a 90 degree hybrid mixer 308. The local oscillator304 provides a signal that is split by beam splitter 313 and componentsof the local oscillator signal are sent to each of 90 degree hybridmixers 306 and 308. The 90 degree hybrid mixers 306 and 308 are opticalcomponents that each generate two phase differentiated optical signals,the XI and XQ signals and the YI and YQ signals, respectively. Finally,each of the four phase differentiated signals are converted toelectrical signals by respective photodiodes and electrical amplifiers(collectively indicated by the numerals 316, 316′, 316″ and 316′″). Insome embodiments, the electrical amplifiers are transimpedanceamplifiers, but in principle other kinds of amplifiers can also be used.The electrical signals are then provided at four respective outputterminals (which may be single-sided signals referenced to a commonground or may be differential signals). Because skew is generated duringthe process of converting the input optical signal into the outputelectrical signals XI, XQ, YI and YQ; as well as in the length ofelectrical wiring due to the fan-out from the circuit to the packagepins, post-compensation for the skew is then applied as shown in FIG.3A. In principle, the systems and methods of the invention can also beapplied to signals that are differentiated in amplitude or frequency

In the systems and methods of the present invention, optical delay linesare used in order to pre-compensate for any electrical-domain skews inthe optical signal paths. Optical delay lines can be integrated onto thesame photonic integrated circuit that performs polarization splittingand the 90° mixing without enlarging the size of the chip. It isbelieved that an advantage of eliminating the need for electrical skewcompensation is a reduction in the size of the larger package. Inaddition, optical compensation delay lines can be used to compensate forskews outside the package in any of the components and in wiring betweena signal source or receiver, in either the transmit or receive path.

FIG. 4 is a diagram 400 that shows an embodiment of the reduced packagesize in a packaged optical receiver enabled by moving skew compensationfrom the electrical domain to the optical domain. The optical domainintroduces timing skew that pre-corrects for the skew introduced later.The pre-correction is accomplished by applying a skew of the oppositesense to a signal that will suffer skew during the optical to electricalconversion process, so that the net skew for that signal is zero, or isas close to zero as is practical. As shown in FIG. 4, the packagedoptical receiver 420 has all of the components present and described inin the packaged optical receiver 320 of FIG. 3B. In addition, thepackaged optical receiver 420 has skew compensation elements 410, 410′,410″ and 410′″ (which can be optical delay lines) situated after theoutputs of the X-Pol and Y-Pol 90 degree hybrid mixers 306 and 308.

In the systems and methods of the present invention, optical delay linesare used in order to pre-compensate for any electrical-domain skews inthe optical signal paths. However, it may also be advantageous toincrease the skew or reduce the skew to some other non-zero skew for thepurposes of constructing a feed-forward, feed-backward, or equalizingfilter.

It is believed that in various embodiments, the optical delay lines canbe implemented using silicon optical waveguides on the same substrate asother optical and electro-optical components in the receiver path.Silicon waveguides can be very tightly confining, and delay lines up tomany picoseconds can be accommodated without any impact on the totalarea requirement of the photonic integrated circuit.

While FIG. 4 shows an embodiment in which a skew compensation element isprovided on each input of the 90 degree hybrid mixers 306 and 308, itshould be understood that the skew compensation described herein can beimplemented in a simpler, less capable system having only two electricaloutput components, by providing only one 90 degree hybrid mixer and onlyone skew compensation element in optical communication with one of thetwo optical inputs of the one 90 degree hybrid mixer. Such a systemwould not be effective in a full dual polarization coherentcommunication system in all instances, but it would be effective for aless capable coherent communication system having only two electricaloutput components.

While FIG. 4 shows an embodiment of a packaged optical receiver thatcomprises pre-skew compensation, it should be understood that one canequally provide the “mirror image” optical compensation for skew in apackaged optical transmitter. This may be readily envisioned byreversing the sense of the electrical signals to input signals in FIG.4, reversing the sense of the electrical amplifiers 316, 316′, 316″ and316″′ by replacing them with optical modulators, replacing the 90 degreehybrid mixers with polarization rotators and splitters (PSRs) asdescribed in co-pending U.S. provisional patent application No.62/118,420 and in co-pending U.S. provisional patent application No.62/132,742 (which polarization rotators and splitters are operated inthe combining sense), and applying the skew correction described to theoutputs of the PSRs before combining all the signals and providing themas output at an output port (e.g., the converse of the input port 302).Thereby providing an optical transmitter with optical skew compensation.

FIG. 5 is a diagram that shows an embodiment of the reduced package sizeenabled by moving skew compensation from the electrical domain to theoptical domain. In some embodiments, the optical domain introducestiming skew that pre-corrects for the skew introduced later. In FIG. 5,in the XI portion of the receiver, a skew compensation element (510,511) is provided between the 90 degree hybrid mixer 306 and theelectrical amplifier 316. Similar skew compensation elements are alsoshown in each of the other XQ, YI and YQ portions of the receiver. Thecomponents of the embodiment of FIG. 5 that are not explicitlyidentified with numerals are the equivalents of the correspondingcomponents shown in FIG. 3 and FIG. 4.

FIG. 6 is a schematic diagram 600 of an embodiment in which a signalthat requires a skew compensation which may vary with time or withfrequency can be skew pre-compensated using one of N different skewcompensation elements. In FIG. 6, the optical signal enters on inputport 610 and is switched by 1×N optical switch 620 to a respective oneof N different skew compensation elements (631, 632, . . . , 63N) andthen is switched by Nxl optical switch 640 to an output port 650. Aslong as 1×N optical switch 620 and N×1 optical switch 640 are operatedto connect the same skew compensation element between the input port 610and the output port 650 at any given time, the embodiment of FIG. 6 canbe used to provide a selected one of N different skew compensationvalues to an optical signal.

FIG. 7 is a schematic diagram 700 that illustrates the generalprinciples of the invention. In the generic embodiment illustrated inFIG. 7, a signal input port (input signal waveguide) 710 and a referencesignal input port 712 (LO or local oscillator waveguide) are provided.The two signals are processed in an optical hybrid element 720 togenerate optical signals having different components. Optical skewcompensation elements 730, 732, 734, 736 are provided to apply apre-skew to each optical component. Opto-electronic conversion elements740, 742 convert the optical signals into electrical signals that aretransmitted and that will experience skews 750, 752, 754 and 756 duringthe electrical transmission. In FIG. 7, the electrical skews 750 and 752are shown using the same schematic elements as pre-skews 732 and 730,respectively, which is intended to indicate that the pre-skew 730 whenadded to the skew 750 is the same total skew as the sum of pre-skew 732and skew 752. Similarly, FIG. 7 is intended to indicate that the sum ofpre-skew 734 and skew 754 equals the sum of pre-skew 736 and skew 756.By such skew compensation, the signals on the respective pairs oftransmission lines arrive at their destination with zero relative skewto each other.

FIG. 8 is a schematic diagram 800 of an embodiment of a system having Nchannels in parallel in which skew compensation in provided in atransmitter.

In FIG. 8, the first channel has an electrical input 812, an electricaltransmission medium 810, an optical transmitter 820 comprising anelectro-optic converter 821 and a skew compensation element 815, anoptical transmission medium 830, an optical receiver 840 and anelectrical transmission medium 850 that provides an electrical signal atan electrical output port 819.

Channels 2, . . . , N have substantially identical elements 810, 820,830, 840 and 850 as are present in Channel 1. However, each respectivechannel 2, . . . , N has a respective electrical input 822, . . . , 8N2,a respective skew compensation element 825, . . . , 8N5, and arespective electrical output port 829, . . . , 8N9.

Skew between channels 1, 2, . . . , N may be introduced in propagationthrough the transmission medium, the optical receiver, and theelectrical transmission medium. The skew may be pre-compensated in theoptical transmitter for the skews introduced in the aforementionedsources. The skews introduced may be a function of frequency. In someembodiments, the net skew introduced by the aforementioned sources ispre-compensated in the optical transmitter.

FIG. 9 a schematic diagram 900 of an embodiment having N channels inparallel in which skew compensation in provided in a receiver.

In FIG. 9, the first channel has an electrical input 912, an electricaltransmission medium 910, an optical transmitter 920, an opticaltransmission medium 930, an optical receiver 940 comprising a skewcompensation element 915 and an electro-optic converter 942 and anelectrical transmission medium 950 that provides an electrical signal atan electrical output port 919.

Channels 2, . . . , N have substantially identical elements 910, 920,930, 940 and 950 as are present in Channel 1. However, each respectivechannel 2, . . . , N has a respective electrical input 922, . . . , 9N2,a respective skew compensation element 925, . . . , 9N5, and arespective electrical output port 929, . . . , 9N9.

Skew between channels 1, 2, . . . , N may be introduced in propagationthrough the transmission medium, the optical transmitter, and theelectrical transmission medium. The skew may be pre-compensated in theoptical receiver for the skews introduced in the aforementioned sources.The skews introduced may be a function of frequency. In someembodiments, the net skew introduced by the aforementioned sources ispre-compensated in the optical receiver.

FIG. 10 is a schematic diagram 1000 of an embodiment in which N channelseach having 4 electrical inputs and 4 electrical outputs are correctedfor skew in a skew compensating module comprising a polarizingbeamsplitter, one or more hybrid mixers, the skew compensation elementsand one or more optoelectronic converters.

In the embodiment of FIG. 10, each of N channels includes a 4×Nelectrical input 1005, a first electrical transmission medium 1010, anoptical transmitter 1020, an optical transmission medium 1030, a skewcompensating module 1040 comprising a polarizing beamsplitter (PBS)1042, one or more hybrid mixers 1044, the skew compensation elements1046 and one or more optoelectronic converters 1048, and a firstelectrical transmission medium 1050 that sends signals out through a 4×Nelectrical output 1055.

Skew between channels 1, 2, . . . , N may be introduced in propagationthrough the first electrical transmission medium 1010, the opticaltransmitter 1020, and the optical transmission medium 1030. The skew maybe pre-compensated in the skew compensating module 1040 for the skewsintroduced in the aforementioned sources. The skews introduced may be afunction of frequency. In some embodiments, the net skew introduced bythe aforementioned sources is pre-compensated in the skew compensatingmodule 1040. For phase-differentiated signals, the skews need to becompensated after the hybrid mixer 1044.

Skew Compensation Embodiments Silicon Single-Mode Waveguides for ShortSkew Compensation

In one embodiment, a 500 nm width and 220 nm height silicon waveguideclad in oxide approximately 75 μm of length corresponds to 1 picosecondof delay in the optical signal passing through the waveguide. This typeof waveguide has on the order of 1 to 2 dB of optical loss percentimeter. Thus, relatively short skews of a few picoseconds can becompensated with a single-mode waveguide without significant excessloss.

Wide Multi-Mode Waveguides for Large Skew Compensation

In other embodiments, 1.2 μm width by 220 nm height silicon waveguidesclad in oxide are multi-modal for illumination at 1550 nm wavelength,but can be adiabatically coupled into from single mode waveguides. Thelowest propagation mode of wide waveguides typically has a very lowinsertion loss, typically on the order of 0.1 to 0.5 dB per centimeter.Thus, these types of waveguides are ideal for compensating large amountsof skew.

Periodic Mode Throttlers for Spectral Smoothness

A common problem in long waveguides is ripples that appear in thetransmission spectrum. These ripples are caused in part byback-reflected light in higher order modes. A mode throttle is awaveguide-integrated device that passes the lowest order mode andattenuates higher order modes. If a long waveguide section has periodicmode throttlers integrated therein, the transmission spectrum may besmoothed. Thus, in some embodiments, the need for skew compensation isalleviated with the use of periodic mode throttlers in applications orsystems that use both single- and multi-mode waveguides. The design andimplementation of mode throttlers is described in greater detail inco-pending U.S., patent application Ser. No. 14/788,608, now U.S. patentapplication Publication Ser. No. ______.

Silicon Nitride Waveguides in the Front-End and Back-End Stack

Silicon nitride is another material that can be integrated on a SOIplatform. Single-mode waveguides can be built in SiN and coupled to andfrom single-mode waveguides in silicon. It is believed that in variousembodiments, these waveguides can also be used for skew compensation.

Additionally, it is possible to use the silicon nitride layers higher inthe metal stack for optical routing. This is described in greater detailin co-pending U.S. patent application Ser. No. 14/798,780, published asU.S. patent application Publication Ser. No. ______. Similarly, it isbelieved that these waveguides may be used for skew compensation invarious embodiments.

Tunable Skew Compensation

It is often desirable to have variable skew compensation. The opticalpath length of a silicon waveguide can be adjusted by integratingheating resistors next to or in the waveguide. It is believed that longrunouts of multi-mode waveguides with heaters can be used to create avery large tuning range. In some embodiments, a thermal measurementdevice is provided, whether a pn junction, a photodetector, anelectro-absorption modulator, or some other electro-optical device. Thethermal measurement device may be any convenient device. In someembodiments the thermal measurement device is a Proportional to AbsoluteTemperature (PTAT) device. Examples of prior art heaters and PTATcircuits are described in co-pending U.S. patent application Ser. No.14/864,760, published as U.S. patent application Publication Ser. No.______, and in U.S. Pat. No. 8,274,021, and are believed to be suitablefor use in the present invention.

In some embodiments, it is believed that it is possible to use thesystems and methods described herein to increase the skew between twosignals, for example for purposes of signal processing.

Feedforward and Feedback Control

In some embodiments, a feedback loop and/or a feed forward loop isprovided to control skew observed between two signals. For example in afeedback control system, one can measure the net skew and control thecorrective delay to achieve a desired amount of skew. In a feedforwardsystem, if one has experience with specific circuits or devices and hasa reasonable expectation of the uncorrected skew that may be expected,one can apply a compensation by way of a corrective delay to achieve anexpected net skew, in the absence of making a measurement of the skew,either before or after the corrective delay is applied. Both feedbackand feed-forward loops used to control or regulate signals are wellknown in the art.

Switched Delay Lines

An even larger distribution of skews can be accommodated through the useof switched delay lines. A 1×N electro-optic switch can be used toswitch between N different sets of waveguide lengths. Furthermore, eachindividual waveguide runout within the switch may have a tunable lengthas described hereinabove to provide a continuously tunable large delayadjustment.

Operating Ranges

It is believed that apparatus constructed using principles of theinvention and methods that operate according to principles of theinvention can be used in the wavelength ranges described in Table I.

TABLE I Band Description Wavelength Range O band original 1260 to 1360nm E band extended 1360 to 1460 nm S band short wavelengths 1460 to 1530nm C band conventional (“erbium window”) 1530 to 1565 nm L band longwavelengths 1565 to 1625 nm U band ultralong wavelengths 1625 to 1675 nm

It is believed that in various embodiments, apparatus as previouslydescribed herein can be fabricated that are able to operate at awavelength within the range of a selected one of an O-Band, an E-band, aC-band, an L-Band, an S-Band and a U-band.

It is believed that apparatus constructed using principles of theinvention and methods that operate according to principles of theinvention can be fabricated using materials systems other than siliconor silicon on insulator. Examples of materials systems that can be usedinclude materials such as compound semiconductors fabricated fromelements in Groups III and V of the Periodic Table (e.g., compoundsemiconductors such as GaAs, AlAs, GaN, GaP, InP, and alloys and dopedcompositions thereof).

Design and Fabrication

Methods of designing and fabricating devices having elements similar tothose described herein, including high index contrast siliconwaveguides, are described in one or more of U.S. Pat. Nos. 7,200,308,7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970, 7,894,696,8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102, 8,258,476,8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016, 8,390,922,8,798,406, and 8,818,141.

DEFINITIONS

As used herein, the term “optical communication channel” is intended todenote a single optical channel, such as light that can carryinformation using a specific carrier wavelength in a wavelength divisionmultiplexed (WDM) system.

As used herein, the term “optical carrier” is intended to denote amedium or a structure through which any number of optical signalsincluding WDM signals can propagate, which by way of example can includegases such as air, a void such as a vacuum or extraterrestrial space,and structures such as optical fibers and optical waveguides.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

INCORPORATION BY REFERENCE

Any material, or portion thereof, that is said to be incorporated byreference herein, but which conflicts with existing definitions,statements, or other disclosure material explicitly set forth herein isonly incorporated to the extent that no conflict arises between thatincorporated material and the present disclosure material. In the eventof a conflict, the conflict is to be resolved in favor of the presentdisclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A skew compensation apparatus, comprising: asignal converter selected from the group of signal converters consistingof a signal converter configured to convert at least two optical signalsinto at least two electrical signals and a signal converter configuredto convert at least two electrical signals into at least two opticalsignals; a first one of said at least two electrical signals subject toa delay of magnitude T_(skew) relative to a second one of said at leasttwo electrical signals; said signal converter having at least twooptical ports and at least two electrical ports, said at least twooptical ports selected from the group consisting of at least two inputports and at least two output ports, and said at least two electricalports selected from the other type of port in the group consisting of atleast two input ports and at least two output ports; and at least oneoptical delay line in optical communication with at least one of said atleast two optical ports, said at least one optical delay line configuredto apply a correction comprising a compensation delay to a selected oneof said first one and said second one of said at least two electricalsignals so that after said correction, the time delay between said firstone and said second one of said at least two electrical signals isdifferent from T_(skew).
 2. The skew compensation apparatus of claim 1,wherein the delay after correction is less than T_(skew).
 3. The skewcompensation apparatus of claim 1, wherein the delay after correction isgreater than T_(skew).
 4. The skew compensation apparatus of claim 1,wherein said at least two optical signals are phase differentiated andcomprise an I component and a Q component.
 5. The skew compensationapparatus of claim 1, wherein said at least two optical signals areconverted from orthogonal polarizations in an optical carrier.
 6. Theskew compensation apparatus of claim 1, wherein said at least oneoptical delay line is a single mode waveguide.
 7. The skew compensationapparatus of claim 1, wherein said at least one optical delay line is amulti-mode waveguide.
 8. The skew compensation apparatus of claim 1,wherein said at least one optical delay line has an adjustable opticalpath length.
 9. The skew compensation apparatus of claim 8, wherein saidadjustable optical path length is configured to be thermally adjustable.10. The skew compensation apparatus of claim 8, wherein said adjustableoptical path length is configured to be adjustable by charge carrierconcentration.
 11. The skew compensation apparatus of claim 1, whereinsaid at least one optical delay line comprises silicon.
 12. The skewcompensation apparatus of claim 1, wherein said at least one opticaldelay line is a switched delay line.
 13. The skew compensation apparatusof claim 1, wherein said at least one optical delay line is a 1×Nelectro-optic switch combined with N waveguides having differentlengths.
 14. The skew compensation apparatus of claim 1, furthercomprising a thermal measurement device and a heater adjacent saidoptical delay line.
 15. The skew compensation apparatus of claim 1,configured to operate using an optical signal having a wavelength withinthe range of a selected one of an O-Band, an E-band, a C-band, anL-Band, an S-Band and a U-band.
 16. A method of compensating skew,comprising the steps of: providing an apparatus, comprising: a signalconverter selected from the group of signal converters consisting of asignal converter configured to convert at least two optical signals intoat least two electrical signals and a signal converter configured toconvert at least two electrical signals into at least two opticalsignals; a first one of said at least two electrical signals subject toa delay of magnitude T_(skew) relative to a second one of said at leasttwo electrical signals; said signal converter having at least twooptical ports and at least two electrical ports, said at least twooptical ports selected from the group consisting of at least two inputports and at least two output ports, and said at least two electricalports selected from the other type of port in the group consisting of atleast two input ports and at least two output ports; and at least oneoptical delay line in optical communication with at least one of said atleast two optical port, said at least one optical delay line configuredto apply a correction comprising a compensation delay to a selected oneof said first one and said second one of said at least two electricalsignals so that after said correction, the time delay between said firstone and said second one of said at least two electrical signals isdifferent from T_(skew); and applying at least two input signals to saidat least two input ports of said signal converter; and applying saidcompensation delay to a selected one of said first one and said secondone of said at least two electrical signals so that said time delaybetween said first one and said second one of said at least twoelectrical signals is different from T_(skew).
 17. The method ofcompensating skew of claim 16, wherein the delay after correction isless than T_(skew).
 18. The method of compensating skew of claim 16,wherein the delay after correction is greater than T_(skew).
 19. Themethod of compensating skew of claim 16, further comprising the step ofdetermining said magnitude T_(skew) of said delay.
 20. The method ofcompensating skew of claim 16, wherein said at least two optical signalsare phase-differentiated and comprise an I component and a Q component.21. The method of compensating skew of claim 16, wherein said at leasttwo optical signals are converted from orthogonal polarizations in theoptical carrier.
 22. The method of compensating skew of claim 16,wherein said at least one optical delay line is a single mode waveguide.23. The method of compensating skew of claim 16, wherein said at leastone optical delay line is a multi-mode waveguide.
 24. The method ofcompensating skew of claim 16, wherein said at least one optical delayline has an adjustable optical path length.
 25. The method ofcompensating skew in an optical system of claim 24, wherein saidadjustable optical path length is configured to be thermally adjustable.26. The method of compensating skew of claim 24, wherein said adjustableoptical path length is configured to be adjustable by charge carrierconcentration.
 27. The method of compensating skew of claim 16, whereinsaid at least one optical delay line comprises silicon.
 28. The methodof compensating skew of claim 16, wherein said at least one opticaldelay line is a switched delay line.
 29. The method of compensating skewof claim 16, wherein said at least one optical delay line is a 1×Nelectro-optic switch combined with N waveguides having differentlengths.
 30. The method of compensating skew of claim 16, furthercomprising a thermal measurement device and a heater adjacent saidoptical delay line.
 31. The method of compensating skew of claim 16,wherein said optical signal has a wavelength within the range of aselected one of an O-Band, an E-band, a C-band, an L-Band, an S-Band anda U-band.